BH250 Aux-in does not work when headphones plugged in: Feature or bug?

I have just purchased a BH250 to carry around and play when in hotels rooms and also in small sessions. As I need to play along to bits and pieces I get sent from fellow musicians its quite handy that the amp comes with a aux-in to connect e.g. with a phone. However I just noticed that while the aux-in is heard on speakers, its not functioning when headphones are plugged in, so you cant listen to the track on the headphones. Is this intentional? If yes. this does not make sense at least to me.